Electrostatic spraying apparatus is described in British patent specification No. 1,569,707, which apparatus comprises a spray head having a conducting or semiconducting surface; means for electrically charging the sprayhead surface to a potential of the order of 1-20 kilovolts; means for delivering spray liquid to the surface; a field-intensifying electrode mounted adjacent to the surface; and means for connecting the field-intensifying electrode to earth, the electrode being so sited relative to the surface that when the surface is charged, the electrostatic field thereat causes liquid thereon to atomise without substantial corona discharge to form electrically charged particles which are projected past the electrode.
In use of such apparatus, as the liquid leaves the surface, it is or becomes charged due to the voltage applied to the surface. The charge produces a repulsive force within the liquid, which overcomes the surface tension thereof causing the liquid to form into one or more cones, sometimes called Taylor cones, dependent on the configuration of the surface. A ligament of liquid is repelled from the tip of the or each cone. The ligament then breaks up into droplets.
The size of the droplets in the spray is distributed over a narrow range compared to other sprayers. The size of the droplets which is obtained depends on the resistivity and the viscosity of the liquid to be sprayed, on the electrical field strength at the spray head, and on the flow rate of the liquid through the spray head. The presence of the field-intensifying electrode adjacent the surface defines the electric field largely independent of the distance of the sprayhead from the target. Therefore the droplet size is largely independent of the distance from the target and can be defined by adjusting the resistivity and the viscosity of the liquid and the voltage applied to the spray head.
The production of a spray of droplets having a narrow size range can be very useful. For example the behaviour of the droplets will be more uniform if the range of sizes is narrow. There are, however, applications when the fact that the droplets are charged is unwanted. British patent specification No. 2,018,627B describes a system for producing a spray which is at least partially discharged. To do this an earthed spike is introduced near the sprayhead. In use an ionic discharge is induced from the spike which discharges the spray at least partially.
A particular application in which it would be useful to produce a non-charged spray of droplets having a narrow distribution of size, is in inhalers to administer drugs to a patient, for example for the treatment of asthma, bronchitis and emphysema. Owing to the fine tubular structure in the lungs the depth to which a particular droplet will penetrate the lungs depends on the size of the droplets. Droplets of the order of 5 microns will reach only the upper respiratory tract, which is quite satisfactory for the treatment of asthma, but for the treatment of emphysema, it is necessary for the droplets to reach the alveoli in the lower respiratory tract, and for this purpose, droplets in the range 0.5 to 2 microns are required. Current state of the art aerosol inhalers produce a wide spectrum of droplet sizes extending up to 37 microns. Current state of the art nebulizers will produce droplets of the required small size, but only in conjunction with droplets of larger sizes, so that with current nebulizers, only a proportion of an anti emphysema drug will reach the required site of action in the alveoli.
Naturally, it is desired that the spray produced be completely discharged for such an application. Charged spray would deposit in the mouth or throat, which would be unpleasant, and would not be inhaled.
In practice, we found it difficult to discharge all of the droplets produced. It was possible, using an earthed needle to discharge a large part of the spray. However, when it was attempted to discharge all the droplets, the ionic discharge (or corona) from the needle reached the cone. This discharged the cone itself which depleted or destroyed the formation of a ligament and thus of a spray.